Molecular characterization of saguaro cactus virus Photographs and illustrations can be viewed by clicking the highlited texts

Molecular characterization of saguaro cactus virus

Zhongguo Xiong and Ziming Weng
Department of Plant Pathology, University of Arizona

Saguaro cactus virus (SCV) is unique to the Sonoran Desert. The only natural host for this small RNA virus is the giant saguaro cactus. Infected cacti do not show any visible symptoms but the impact of SCV on the ecology of the saguaro is not fully understood.

SCV is a member of the genus Carmovirus within the family Tombusviridae. The most closely related virus is carnation mottle virus, a virus frequently found on ornamental plants.

The complete sequence of the small RNA viral genome has been determined. The gene expression of SCV has been characterized by in vitro translation studies of viral subgenomic RNAs and in vitro RNA transcripts. With the infectious cDNA clones that have recently been developed, SCV has become an excellent model system to study viral gene expression and replication.

SCV is a small icosahedral RNA virus. SCV genome consists of a positive-sense, single-stranded RNA of 3879 nucleotides in length. It contains five open reading frames (ORFs), p26, p57, p6, p9, and p36 from the 5' terminus to the 3' terminus. Two (possibly three) encapsidated subgenomic RNAs were identified by primer extension and double-stranded RNA analysis. Five major polypeptides of approximately 26 kd, 57 kd, 6 kd, 9 kd, and 36 kd, corresponding to the five ORFs were observed in the in vitro translation products of purified virion RNA. In addition, an 86 kd polypeptides corresponding to a readthrough product of the first two ORFs was present in the translation products. In vitro translation studies using RNA transcripts of defined lengths showed that only p27 and the p87 readthrough protein were expressed from the genomic RNA. Other proteins encoded by SCV were potentially expressed from subgenomic RNA.

Infectious cDNA clones of SCV have been generated by a reverse transcription-polymerase chain reaction using two primers. The 3' primer consists of an AflII restriction site and 18 nucleotide complementary to the SCV 3' terminal sequence. The 5' primer consists of a XbaI restriction site, promoter sequence for T7 RNA polymerase, and 18 nucleotides corresponding to the 5' terminus of SCV genome. Inoculation of T7 in vitro transcripts resulted in symptoms identical to the wild type viral infection on Chenopodium amaranticolor and C. capitatum.

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